This proposal seeks support for an investigation into the function of a novel gene, Band 17, in growth plate chondrocyte maturation. We identified Band 17 on the basis of differential expression between growth plate and articular chondrocytes, and have found no other tissues in which Band 17 is expressed as well as in growth plate chondrocytes. Band 17expression is significantly higher in growth plate chondrocytes undergoing transition from proliferation to hypertrophy, a pattern of expression supported by immunohistochemistry with anti-Band 17 antibodies. Protein extracts of growth plate chondrocytes, but not extracts of articular chondrocytes, contain proteins of 53-55 kd that are recognized by anti-Band 17 antibodies. The Band 17 locus encodes gene products of 318 and 449 amino acids that are translated from alternatively spliced messages. The 449 amino acid gene product is a C-terminal extension of the 318 amino acid gene product, is at least 10 fold more abundant, and has a consensus sequence at the C-terminal that targets proteins to the lumen of the endoplasmic reticulum. The Band 17 proteins show 21% identity with a group of yeast acid phosphatases, particularly around the active site of the enzyme. This group of phosphatases is in the same family as E. Coli acid phosphatase and human lysosomal and prostatic acid phosphatases. However, based on the overall homology the Band 17 proteins are most homologous to the yeast phosphatases, and therefore may present a previously unknown class of phosphatase in higher eukaryotes. The homology of Band 17 with acid phosphatases is convincing but unproven. Therefore the first specific aims is focused on the biochemical activity and other studies of the Band 17 gene products. The second specific aim will investigate the regulation of Band 17 in chondrocytes in response to external agents, and begin identifying the cis-acting elements in the Band 17 promoter. The third specific aim will examine Band 17 proteins as possible regulatory molecules by underexpressing or overexpressing the transcripts in chondrocytes, then characterizing the resulting phenotype. Our goal is to integrate the information gained from the investigations outlined in the three specific aims. Our studies will provide us with the following data: 1) Whether both or one of the gene products have phosphatase activity, the intra- or extracellular localization of that activity, and the identity of their substrates and/or interacting proteins. 2) The spatial and temporal regulation of the gene and gene products and what elements are determining this pattern of expression. 3) Whether Band 17 expression is sufficient of effect events downstream of its own expression. The distinctive nature of Band 17 and its gene products imply a previously unidentified pathway that is part of, or determines, transition of growth plate chondrocytes from proliferation to hypertrophy. Our work will identify the members of this pathway in addition to Band 17.